The general consensus is that 3 and 4 are the best in the series, but 4 is much more accessible and streamlined. Untold, on the other hand, went too far with that, I think. It added a lot of neat (though not game changing) options for making the game Eben more streamlined than 4, but it also added some that made the difficulty much more uneven. It's really heightened when, as a remake of 1, it still shared a lot of design problems with its predecessor (1 is fun, but it's a VERY broken game). By contrast, 4 has a much nicer learning curve.
Untold also added a story mode with predefined characters, but they're such cardboard cut outs that I really wonder why they bothered. It also prevented you from using any classes aside from the assigned five, which took away your ability to make a party specifically tailored to your tastes. Q looks like what Untold should have been in the first place; each character might be locked to certain skills, but the sheer number of them gives you a ton of versatility.
Either way, you can download demos for 4 and Untold to try 'em out, and their save files will transfer to the full game if you like what you play.
For the record, I've completed all of them except 2, and my personal favorite is 4.